AppVeyor: Build with Visual Studio 2017, cleanup

[EwoutH]

• Updates the build image to and builds with Visual Studio 2017
• Minor cleanup/refactor

[EwoutH]

Currently stuck on the following error:

C:\projects\alicevision\src\aliceVision/prettyprint.hpp(62): fatal error C1001: An internal error has occurred in the compiler. [C:\projects\alicevision\build\src\aliceVision\system\aliceVision_system.vcxproj]
  (compiler file 'd:\agent\_work\8\s\src\vctools\compiler\cxxfe\sl\p1\c\types.c', line 4563)
   To work around this problem, try simplifying or changing the program near the locations listed above.
  Please choose the Technical Support command on the Visual C++ 
   Help menu, or open the Technical Support help file for more information (compiling source file C:\projects\alicevision\src\aliceVision\system\Logger.cpp)

[EwoutH]

c:\program files\nvidia gpu computing toolkit\cuda\v9.2\include\crt/host_config.h(133):
fatal error C1189: #error:
-- unsupported Microsoft Visual Studio version! Only the versions 2012, 2013, 2015 and 2017 are supported!
[C:\projects\alicevision\build\src\aliceVision\depthMap\aliceVision_depthMap.vcxproj]

Now stuck on this error, which is kind of strange, since VS 2017 is used.

Maybe the VS 2017 minor version is too new for the current CUDA version, so I'm going to try updating the patch level.

[EwoutH]

Unfortunately, CUDA 9.2 isn't compatible with Visual Studio 2017 15.9, which AppVeyor provides, but only with VS 2017 15.6 and older. This PR will be succeeded by #776.